Controlling exhaust gas recirculation in a turocharged compression-ignition engine system

ABSTRACT

One embodiment of the invention may include a method of controlling exhaust gas recirculation (EGR) in a turbocharged compression-ignition engine system including a high pressure (HP) EGR path and a low pressure (LP) EGR path. The method may include determining a target total EGR fraction for compliance with exhaust emissions criteria, and determining a target HP/LP EGR ratio to optimize other engine system criteria within the constraints of the determined target total EGR fraction. The determining of the target HP/LP EGR ratio may include using at least engine speed and load as input to a base model to output a base EGR value, using at least one other engine system parameter as input to at least one adjustment model to output at least one EGR adjustment value, and adjusting the base EGR value with the at least one EGR adjustment value to generate at least one adjusted EGR value.

This application claims the benefit of United States ProvisionalApplication No. 60/908,528 filed Mar. 28, 2007.

TECHNICAL FIELD

The field to which the disclosure generally relates includes controllingexhaust gas recirculation within turbocharged compression-ignitionengine systems.

BACKGROUND

Turbocharged engine systems include engines having combustion chambersfor combusting air and fuel for conversion into mechanical power, airinduction subsystems for conveying induction gases to the combustionchambers, and engine exhaust subsystems. The exhaust subsystemstypically carry exhaust gases away from the engine combustion chambers,muffle engine exhaust noise, and reduce exhaust gas particulates andoxides of nitrogen (NOx), which increase as engine combustiontemperatures increase. Exhaust gas is often recirculated out of theexhaust gas subsystem, into the induction subsystem for mixture withfresh air, and back to the engine. Exhaust gas recirculation increasesthe amount of inert gas and concomitantly reduces oxygen in theinduction gases, thereby reducing engine combustion temperatures and,thus, reducing NOx formation.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

One exemplary embodiment includes a method of controlling exhaust gasrecirculation (EGR) in a turbocharged compression-ignition engine systemincluding a high pressure (HP) EGR path and a low pressure (LP) EGRpath. The method may include determining a target total EGR fraction forcompliance with exhaust emissions criteria, and determining a targetHP/LP EGR ratio to optimize other engine system criteria within theconstraints of the determined target total EGR fraction. The determiningof the target HP/LP EGR ratio may include using at least engine speedand load as input to a base model to output a base EGR value, using atleast one other engine system parameter as input to at least oneadjustment model to output at least one EGR adjustment value, andadjusting the base EGR value with the at least one EGR adjustment valueto generate at least one adjusted EGR value.

Another exemplary embodiment includes a method of controlling exhaustgas recirculation (EGR) in a turbocharged compression-ignition enginesystem including a high pressure (HP) EGR path and a low pressure (LP)EGR path. The method may include determining a target total EGR fractionfor compliance with exhaust emissions criteria, and determining a targetHP/LP EGR ratio to optimize other engine system criteria within theconstraints of the determined target total EGR fraction. The determiningof the target HP/LP EGR ratio may include using at least engine speedand load in at least one model to output an LP EGR value and an HP EGRvalue, applying the target total EGR fraction to the LP and HP EGRvalues to establish LP and HP EGR setpoints, and delaying downstreamcommunication of the HP EGR value to account for lag time between LP andHP EGR.

Other exemplary embodiments of the invention will become apparent fromthe following detailed description. It should be understood that thedetailed description and specific examples, while indicating theexemplary embodiment of the invention, are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will become more fullyunderstood from the detailed description and the accompanying drawings,wherein:

FIG. 1 is a schematic view of an exemplary embodiment of an enginesystem including an exemplary control subsystem;

FIG. 2 is a block diagram of the exemplary control subsystem of theengine system of FIG. 1;

FIG. 3 is a flow chart of an exemplary method of EGR control that may beused with the engine system of FIG. 1;

FIG. 4 is a block diagram illustrating a preferred control flow portionof the method of FIG. 3 and including a total EGR estimation block andhigh and low pressure EGR open-loop control blocks;

FIGS. 5A-5C illustrate exemplary embodiments of the estimation block ofFIG. 4;

FIGS. 6A-6B illustrate exemplary embodiments of the high and lowpressure EGR open-loop control blocks of FIG. 4;

FIG. 7 is a graph illustrating an exemplary plot of valve positionversus target total EGR fraction;

FIG. 8 is a block diagram illustrating a second control flow portion ofthe method of FIG. 3;

FIG. 9 a block diagram illustrating a third control flow portion of themethod of FIG. 3;

FIG. 10 is a block diagram illustrating a fourth control flow portion ofthe method of FIG. 3;

FIG. 11 is a block diagram of an exemplary control flow portion of HP/LPEGR ratio optimization and including an HP/LP EGR ratio determinationblock and an HP/LP EGR compensation block;

FIG. 12 is a block diagram of an exemplary control flow portion of theHP/LP EGR ratio determination block of FIG. 11;

FIG. 13 is a block diagram of an exemplary control flow portion of atransient load adjustment model of the HP/LP EGR ratio determinationblock of FIG. 11;

FIG. 14 is a block diagram of an exemplary control flow portion of aninduction temperature adjustment model of the HP/LP EGR ratiodetermination block of FIG. 11; and

FIGS. 15A and 15B are block diagrams of exemplary control flow portionsof a turbocharger protection adjustment model of the HP/LP EGR ratiodetermination block of FIG. 11.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description of the embodiments is merely exemplary innature and is in no way intended to limit the invention, itsapplication, or uses.

Exemplary System

An exemplary operating environment is illustrated in FIG. 1, and may beused to implement a presently disclosed method of EGR control. Themethod may be carried out using any suitable system and, preferably, iscarried out in conjunction with an engine system such as system 10. Thefollowing system description simply provides a brief overview of oneexemplary engine system, but other systems and components not shown herecould also support the presently disclosed method.

In general, the system 10 may include an internal combustion engine 12to develop mechanical power from internal combustion of a mixture offuel and induction gases, an induction subsystem 14 to generally providethe induction gases to the engine 12 and, an exhaust subsystem 16 toconvey combustion gases generally away from the engine 12. The engine 12may be constructed and arranged to combust diesel, gasoline or othercombustible fuels. As used herein, the phrase induction gases mayinclude fresh air and recirculated exhaust gases. The system 10 alsogenerally may include a turbocharger 18 in communication across theexhaust and induction subsystems 14, 16 to compress inlet air to improvecombustion and thereby increase engine output. The system 10 furthergenerally may include an exhaust gas recirculation subsystem 20 acrossthe exhaust and induction subsystems 14, 16 to recirculate exhaust gasesfor mixture with fresh air to improve emissions performance of theengine system 10. The system 10 further generally may include a controlsubsystem 22 to control operation of the engine system 10. Those skilledin the art will recognize that a fuel subsystem (not shown) is used toprovide any suitable liquid and/or gaseous fuel to the engine 12 forcombustion therein with the induction gases.

The internal combustion engine 12 may be any suitable type of engine,such as an autoignition or compression-ignition engine like a dieselengine. The engine 12 may include a block 24 with cylinders and pistonstherein (not separately shown), which along with a cylinder head (alsonot separately shown), define combustion chambers (not shown) forinternal combustion of a mixture of fuel and induction gases.

The induction subsystem 14 may include, in addition to suitable conduitand connectors, an inlet end 26 which may have an air filter (not shown)to filter incoming air, and a turbocharger compressor 28 downstream ofthe inlet end 26 to compress the inlet air. The induction subsystem 14may also include a charge air cooler 30 downstream of the turbochargercompressor 28 to cool the compressed air, and an intake throttle valve32 downstream of the charge air cooler 30 to throttle the flow of thecooled air to the engine 12. The induction subsystem 14 also may includean intake manifold 34 downstream of the throttle valve 32 and upstreamof the engine 12, to receive the throttled air and distribute it to theengine combustion chambers.

The exhaust subsystem 16 may include, in addition to suitable conduitand connectors, an exhaust manifold 36 to collect exhaust gases from thecombustion chambers of the engine 12 and convey them downstream to therest of the exhaust subsystem 16. The exhaust subsystem 16 also mayinclude a turbocharger turbine 38 in downstream communication with theexhaust manifold 36. The turbocharger 18 may be a variable turbinegeometry (VTG) type of turbocharger, a dual stage turbocharger, or aturbocharger with a wastegate or bypass device, or the like. In anycase, the turbocharger 18 and/or any turbocharger accessory device(s)may be adjusted to affect any one or more of the following parameters:turbocharger boost pressure, air mass flow, and/or EGR flow. The exhaustsubsystem 16 may also include any suitable emissions device(s) 40 suchas a catalytic converter like a close-coupled diesel oxidation catalyst(DOC) device, a nitrogen oxide (NOx) adsorber unit, a particulatefilter, or the like. The exhaust subsystem 16 may also include anexhaust throttle valve 42 disposed upstream of an exhaust outlet 44.

The EGR subsystem 20 is preferably a hybrid or dual path EGR subsystemto recirculate portions of the exhaust gases from the exhaust subsystem16 to the induction subsystem 14 for combustion in the engine 12.Accordingly, the EGR subsystem 20 may include two paths: a high pressure(HP) EGR path 46 and a low pressure (LP) EGR path 48. Preferably, the HPEGR path 46 may be connected to the exhaust subsystem 16 upstream of theturbocharger turbine 38 but connected to the induction subsystem 12downstream of the turbocharger compressor 28. Also preferably, the LPEGR path 48 may be connected to the exhaust subsystem 16 downstream ofthe turbocharger turbine 38 but connected to the induction subsystem 14upstream of the turbocharger compressor 28. Any other suitableconnection between the exhaust and induction sub-systems 14, 16 is alsocontemplated including other forms of HP EGR such as the usage ofinternal engine variable valve timing and lift to induce internal HPEGR. According to internal HP EGR, operation of engine exhaust andintake valves may be timed so as to communicate some exhaust gasesgenerated during one combustion event back through intake valves so thatexhaust gases are combusted in a subsequent combustion event.

The HP EGR path 46 may include, in addition to suitable conduit andconnectors, an HP EGR valve 50 to control recirculation of exhaust gasesfrom the exhaust subsystem 16 to the induction subsystem 14. The HP EGRvalve 50 may be a stand-alone device having its own actuator or may beintegrated with the intake throttle valve 32 into a combined devicehaving a common actuator. The HP EGR path 46 may also include an HP EGRcooler 52 upstream, or optionally downstream, of the HP EGR valve 50 tocool the HP EGR gases. The HP EGR path 46 may be connected upstream ofthe turbocharger turbine 38 and downstream of the throttle valve 32 tomix HP EGR gases with throttled air and other induction gases (the airmay have LP EGR).

The LP EGR path 48 may include, in addition to suitable conduit andconnectors, an LP EGR valve 54 to control recirculation of exhaust gasesfrom the exhaust subsystem 16 to the induction subsystem 14. The LP EGRvalve 54 may be a stand-alone device having its own actuator or may beintegrated with the exhaust throttle valve 42 into a combined devicehaving a common actuator. The LP EGR path 48 may also include an LP EGRcooler 56 downstream, or optionally upstream, of the LP EGR valve 54 tocool the LP EGR gases. The LP EGR path 48 may be connected downstream ofthe turbocharger turbine 38 and upstream of the turbocharger compressor28 to mix LP EGR gases with filtered inlet air.

Referring now to FIG. 2, the control subsystem 22 may include anysuitable hardware, software, and/or firmware to carry out at least someportions of the methods disclosed herein. For example, the controlsubsystem 22 may include some or all of the engine system actuators 58discussed above, as well as various engine sensors 60. The engine systemsensors 60 are not individually shown in the drawings but may includeany suitable devices to monitor engine system parameters.

For example, an engine speed sensor may measure the rotational speed ofan engine crankshaft (not shown), pressure sensors in communication withthe engine combustion chambers may measure engine cylinder pressure,intake and exhaust manifold pressure sensors may measure pressure ofgases flowing into and away from the engine cylinders, an inlet air massflow sensor may measure incoming airflow in the induction subsystem 14,and a manifold mass flow sensor may measure flow of induction gases tothe engine 12. In another example, the engine system 10 may include atemperature sensor to measure the temperature of induction gases flowingto the engine cylinders, and a temperature sensor downstream of the airfilter and upstream of the turbocharger compressor 28. In a furtherexample, the engine system 10 may include a speed sensor suitablycoupled to the turbocharger compressor 28 to measure the rotationalspeed thereof. A throttle position sensor, such as an integrated angularposition sensor, may measure the position of the throttle valve 32. Aposition sensor may be disposed in proximity to the turbocharger 18 tomeasure the position of the variable geometry turbine 38. A tailpipetemperature sensor may be placed just upstream of a tailpipe outlet tomeasure the temperature of the exhaust gases exiting the exhaustsubsystem 16. Also, temperature sensors may be placed upstream anddownstream of the emissions device(s) 40 to measure the temperature ofexhaust gases at the inlet(s) and outlet(s) thereof. Similarly, one ormore pressure sensors may be placed across the emissions device(s) 40 tomeasure the pressure drop thereacross. An oxygen (O₂) sensor may beplaced in the exhaust and/or induction subsystems 14, 16, to measureoxygen in the exhaust gases and/or induction gases. Finally, positionsensors may measure the positions of the HP and LP EGR valves 50, 54 andthe exhaust throttle valve 42.

In addition to the sensors 60 discussed herein, any other suitablesensors and their associated parameters may be encompassed by thepresently disclosed system and methods. For example, the sensors 60 mayalso include accelerator sensors, vehicle speed sensors, powertrainspeed sensors, filter sensors, other flow sensors, vibration sensors,knock sensors, intake and exhaust pressure sensors, and/or the like. Inother words, any sensors may be used to sense any suitable physicalparameters including electrical, mechanical, and chemical parameters. Asused herein, the term sensor may include any suitable hardware and/orsoftware used to sense any engine system parameter and/or variouscombinations of such parameters.

The control subsystem 22 may further include one or more controllers(not shown) in communication with the actuators 58 and sensors 60 forreceiving and processing sensor input and transmitting actuator outputsignals. The controller(s) may include one or more suitable processorsand memory devices (not shown). The memory may be configured to providestorage of data and instructions that provides at least some of thefunctionality of the engine system 10 and that may be executed by theprocessor(s). At least portions of the method may be enabled by one ormore computer programs and various engine system data or instructionsstored in memory as look-up tables, formulas, algorithms, maps, models,or the like. In any case, the control subsystem 22 may control enginesystem parameters by receiving input signals from the sensors 60,executing instructions or algorithms in light of sensor input signals,and transmitting suitable output signals to the various actuators 58.

The control subsystem 22 may include one or more modules in thecontroller(s). For example, a top level engine control module 62 mayreceive and process any suitable engine system input signals andcommunicates output signals to an induction control module 64, a fuelcontrol module 66, and any other suitable control modules 68. As will bediscussed in greater detail below, the top level engine control module62 may receive and process input signals from one or more of the enginesystem parameter sensors 60 to estimate total EGR fraction in anysuitable manner. The modules 62, 64, 66, 68, may be separate as shown,or may be integrated or combined into one or more modules, which mayinclude any suitable hardware, software, and/or firmware.

Various methods of estimating EGR fraction are known to those skilled inthe art. As used herein, the phrase “total EGR fraction” may include oneor more of its constituent parameters, and may be represented by thefollowing equation:

$r_{EGR} = {{\left( {1 - \frac{M\; A\; F}{M_{ENG}}} \right)*100} = {\left( \frac{M_{EGR}}{M_{ENG}} \right)*100}}$

where

-   -   MAF is fresh air mass flow into an induction subsystem,    -   M_(EGR) is EGR mass flow into the induction subsystem,    -   M_(ENG) is induction gas mass flow to an engine, and    -   r_(EGR) includes that portion of induction gases entering an        engine attributable to recirculated exhaust gases.

From the above equation, the total EGR fraction may be calculated usingthe fresh air mass flow sensor and induction gas mass flow from a sensoror from an estimate thereof, or using an estimate of total EGR fractionitself and the induction gas mass flow. In either case, the top levelengine control module 62 may include suitable data inputs to estimatethe total EGR fraction directly from one or more mass flow sensormeasurements or estimations as input to one or more engine systemmodels.

As used herein, the term “model” may include any construct thatrepresents something using variables, such as a look up table, map,formula, algorithm and/or the like. Models may be application specificand particular to the exact design and performance specifications of anygiven engine system. In one example, the engine system models in turnmay be based on engine speed and intake manifold pressure andtemperature. The engine system models may be updated each time engineparameters change, and may be multi-dimensional look up tables usinginputs including engine speed and engine intake gas density, which maybe determined with the intake pressure, temperature, and universal gasconstant.

The total EGR fraction may be correlated, directly or indirectly via itsconstituents, to one or more engine system parameters, such as estimatedor sensed air mass flow, O₂, or engine system temperature(s). Suchparameters may be analyzed in any suitable fashion for correlation withthe total EGR fraction. For example, the total EGR fraction may beformulaically related to the other engine system parameters. In anotherexample, from engine calibration or modeling, the total EGR fraction maybe empirically and statistically related to the other engine systemparameters. In any case, where the total EGR fraction is found toreliably correlate to any other engine system parameter(s), thatcorrelation may be modeled formulaically, empirically, acoustically,and/or the like. For example, empirical models may be developed fromsuitable testing and may include lookup tables, maps, formulas,algorithm, or the like that may be processed in the total EGR fractionvalues with other engine system parameter values.

Accordingly, an engine system parameter may be used as a proxy fordirect sensor measurements of total EGR fraction and/or individual HPand/or LP EGR flow. Accordingly, total EGR, HP EGR, and LP EGR flowsensors may be eliminated, thereby saving on engine system cost andweight. Elimination of such sensors also leads to elimination of othersensor-related hardware, software, and costs, such as wiring, connectorpins, computer processing power and memory, and so on.

Also, the top level engine control 62 module may calculate aturbocharger boost pressure setpoint and a target total EGR setpoint,and transmit these setpoints to the induction control module 64.Similarly, the top level engine control module 62 may calculate suitabletiming and fueling setpoints and transmit them to the fuel controlmodule 66, and may calculate other setpoints and transmit them to theother control modules 68. The fuel and other control modules 66, 68 mayreceive and process such inputs, and may generate suitable commandsignals to any suitable engine system devices such as fuel injectors,fuel pumps, or other devices.

Alternatively, the top level engine control module 62 may calculate andtransmit the boost pressure setpoint and a total intake air mass flowsetpoint (as shown in dashed lines), instead of the target total EGRsetpoint. In this alternative case, the total EGR setpoint may besubsequently determined from the air mass flow setpoint in much the sameway the actual total EGR fraction is estimated from the actual mass flowsensor readings. In a second alternative, air mass flow may replacetotal EGR fraction throughout the control method. This changes the typesof data used and the manner in which HP and LP EGR flow targets are set,but the basic structure of the controller and flow of the control methodis the same.

The induction control module 64 may receive any suitable engine systemparameter values, in addition to the setpoints received from the toplevel engine control module 62. For example, the induction controlmodule 64 may receive induction and/or exhaust subsystem parametervalues like turbocharger boost pressure, and mass flow. The inductioncontrol module 64 may include a top level induction control submodule 70that may process the received parameter values, and transmit anysuitable outputs such as LP and HP EGR setpoints and turbochargersetpoints to respective LP EGR, HP EGR, and turbocharger controlsubmodules 72, 74, 76. The LP EGR, HP EGR, and turbocharger controlsubmodules 72, 74, 76 may process such induction control submoduleoutputs and may generate suitable command signals to various enginesystem devices such as the LP EGR valve 54 and exhaust throttle valve42, HP EGR valve 50 and intake throttle valve 32, and one or moreturbocharger actuators 19. The various modules and/or submodules may beseparate as shown, or may be integrated into one or more combinedmodules and/or submodules.

Exemplary Method(s)

A method of controlling LP and HP EGR is provided herein and may becarried out as one or more computer programs within the operatingenvironment of the engine system 10 described above. Those skilled inthe art will also recognize that the method may be carried out usingother engine systems within other operating environments. Referring nowto FIG. 3, an exemplary method 300 is illustrated in flow chart form.

As shown at step 305, the method 300 may be initiated in any suitablemanner. For example, the method 300 may be initiated at startup of theengine 12 of the engine system 10 of FIG. 1.

At step 310, fresh air may be drawn into an induction subsystem of anengine system, and induction gases are inducted into an engine of theengine system through the induction subsystem. For example, fresh airmay be drawn into the inlet 26 of the induction system 14, and inductiongases may be inducted into the engine 12 through the intake manifold 34.

At step 315, exhaust gases may be exhausted from an engine through anexhaust subsystem of an engine system. For example, exhaust gases may beexhausted from the engine 12 through the exhaust manifold 36.

At step 320, exhaust gases may be recirculated from an exhaust subsystemthrough one or both of high or low pressure EGR paths to an inductionsubsystem of an engine system. For example, HP and LP exhaust gases maybe recirculated from the exhaust subsystem 16, through the HP and LP EGRpaths 46, 48, to the induction subsystem 14.

At step 325, one or more proxy parameters may be sensed that is/areindicative of total EGR fraction. For example, the proxy parameter(s)may include air mass flow, O₂, and/or engine system temperatures, andmay be measured by respective sensors 60 of the engine system 10.

At step 330, a target total EGR fraction may be determined forcompliance with exhaust emissions criteria. For example, the top levelengine control module 62 may use any suitable engine system model(s) tocross-reference current engine operating parameters with desirable totalEGR fraction values to comply with predetermined emissions standards. Asused herein, the term “target” includes a single value, multiple values,and/or any range of values. Also, as used herein, the term “criteria”includes the singular and the plural. Examples of criteria used todetermine appropriate EGR fraction(s) include calibrated tables based onspeed and load, model based approaches which determine cylindertemperatures targets and convert to EGR fraction and operatingconditions such as transient operation or steady state operation.Absolute emissions criteria may be dictated by environmental entitiessuch as the U.S. Environmental Protection Agency (EPA).

At step 335, a target HP/LP EGR ratio may be determined to optimize oneor more other engine system criteria such as fuel economy goals, enginesystem performance goals, or engine system protection or maintenancespecifications, and as constrained by the target total EGR fractiondetermined in step 330. Those skilled in the art will appreciate that atarget ratio may be determined by determining one, the other, or both ofthe constituents of the ratio. For example, the target HP/LP EGR ratiomay be determined by determining the HP EGR percentage, the LP EGRpercentage, or both. In any case, step 335 may be carried out inconjunction with FIGS. 11-15 and the accompanying description set forthherein below.

At step 340, individual HP EGR and/or LP EGR setpoints may be generatedin accordance with the target HP/LP EGR ratio determined in step 335.

At step 345, target HP and LP EGR opening percentages corresponding tothe HP and LP EGR setpoints may be determined. For example, open-loopcontrollers may process the HP and LP EGR setpoints and other enginesystem parameters using models to generate the opening percentages.

At step 350, total EGR fraction may be estimated responsive to the proxyparameter(s), which are used as input to any suitable engine systemmodels as discussed previously above. For example, the total EGRfraction estimate may include engine system models to formulaically orempirically correlate the proxy parameter(s) to the total EGR fraction.The models may include lookup tables, maps, and the like, that may crossreference EGR fraction values with proxy parameter values, and may bebased on engine speed and intake manifold pressure and temperature. Inany case, the total EGR fraction is not actually directly measured usingindividual HP and/or LP EGR flow sensors or a combined total EGR flowsensor.

At step 355, one or both of the individual HP EGR and/or LP EGRfractions may be adjusted using closed-loop control with the estimatedtotal EGR fraction. The HP and/or LP EGR fractions may be adjusted viaclosed-loop control of either or both of the respective HP and/or LP EGRsetpoints or the valve and/or throttle opening percentages. For example,and as will be discussed in greater detail below, a closed-loopcontroller may process the estimated total EGR fraction as processvariable input and the total EGR fraction setpoint as a setpoint input,in order to generate an HP and/or LP EGR setpoint output trim command.Thus, the target total EGR fraction preferably is closed-loop controlledby closed-loop adjustments to the HP and/or LP EGR fractions. Suchadjustments may change the actual HP/LP EGR ratio.

At step 360, the HP EGR and LP EGR opening percentages from step 350 maybe applied to one or more respective HP EGR, LP EGR, intake throttle, orexhaust throttle valves. The HP and/or LP EGR opening percentages areadjusted directly, downstream of the open-loop control blocks orindirectly via setpoint adjustment upstream of the open-loop controlblocks.

Exemplary Control Flows

Referring now to the controls diagram of FIG. 4, a portion of thecontrol method 300 from FIG. 3 is illustrated in block form as an EGRcontrol flow 400. The control flow 400 may be carried out, for example,within the exemplary control subsystem of FIG. 2 and, more particularly,within the induction control module 64 thereof. Accordingly, FIG. 4illustrates the HP and LP EGR control submodules or blocks 72, 74 andthe turbocharger boost control submodule or block 76. Similarly, anoptimization block 402, an EGR fraction estimator block 404, and an EGRfraction closed-loop control block 406 may also be carried out withinthe induction control module 64 and, more particularly, within the toplevel induction control submodule 70 of FIG. 2.

First, and referring also to FIGS. 5A-5C, the actual total EGR fractionestimator block 404 may be carried out using the proxy parameter(s) forthe actual total EGR fraction in addition to other standard enginesystem parameters such as engine load, engine speed, turbocharger boostpressure, and/or engine system temperatures. For example, FIG. 5Aillustrates that the proxy parameter may be air mass flow 414 a, whichmay be obtained from any suitable air mass flow estimate or reading suchas from the intake air mass flow sensor. In another example, FIG. 5Billustrates that the proxy parameter may be oxygen percentage 414 b,such as from an O₂ sensor like the O₂ sensor disposed in the inductionsubsystem 14. For instance, the O₂ sensor may be a universal exhaust gasoxygen sensor (UEGO), which may be located in the intake manifold 34. Ina further example, FIG. 5C illustrates that the proxy parameter may beinduction subsystem and exhaust subsystem temperature 414 c taken fromtemperature sensors. For instance, inlet air temperature may be usedsuch as from the air inlet temperature sensor, exhaust temperature suchas from the exhaust temperature sensor, and manifold temperature such asfrom the intake manifold temperature sensor. In all of theabove-approaches, the actual total EGR fraction 416 may be estimatedfrom one or more proxy parameter types.

Second, and referring again to FIG. 4, the optimization block 402 mayreceive and process various engine system inputs to identify an optimalHP/LP EGR ratio and generate an HP EGR setpoint according to that ratio.For example, the optimization block 402 may receive the engine loadsignal 407 and the engine speed signal 408, such as from correspondingsensors in the engine system 10. The engine load signal 407 may includeany parameters such as manifold pressure, fuel injection flow, etc. Theoptimization block 402 may also receive a total EGR fraction setpoint418 such as from the top level engine control module 62. Theoptimization block 402 may be carried out in conjunction with FIGS.11-15 and the accompanying description set forth herein below.

The optimization block 402 may prioritize fuel economy criteria foridentifying the optimal HP/LP EGR ratio and generating the correspondingHP EGR setpoint. According to fuel economy optimization, theoptimization block 402 may include any suitable net turbochargerefficiency model that encompasses various parameters such as pumpinglosses, and turbine and compressor efficiencies. The efficiency modelmay include a principles based mathematical representation of the engineinduction subsystem 14, a set of engine system calibration tables, orthe like. Example criteria used to determine desired EGR ratios to meetfuel economy criteria may include setting a ratio that allows the totalEGR fraction to be achieved without the need for closing the intake orexhaust throttles, which closing tends to negatively impact fueleconomy, or the ratio may be adjusted to achieve an optimal inductiontemperature for maximum fuel economy.

The optimization block 402 may also override the fuel economy criteriato instead optimize other engine system criteria for any suitablepurpose. For example, the fuel economy criteria may be overridden toprovide an HP/LP EGR ratio that provides improved engine systemperformance, such as increased torque output in response to driverdemand for vehicle acceleration. In this case, the controller may favora higher percentage of LP EGR which allows better turbocharger speed-upto reduce turbo lag. In another example, the override may provide adifferent HP/LP EGR ratio to protect the engine system 10 such as toavoid a turbocharger overspeed condition or excess compressor tiptemperatures, or to reduce turbocharger condensate formation, or thelike. In a further example, the override may provide another HP/LP EGRratio to maintain the engine system 10 such as by affecting induction orexhaust subsystem temperatures. For instance, exhaust subsystemtemperatures may be increased to regenerate a diesel particulate filter,and induction temperatures may be reduced to cool the engine 12. As afurther example, induction temperature may be controlled to reduce thepotential for water condensate to form in the inlet induction path.

In any case, the optimization block 402 processes the inputs inaccordance with its model(s) to determine the target HP/LP EGR ratio andthen generate an HP EGR setpoint 420, which is fed downstream to the HPEGR control block 74 and to an arithmetic node 422, which also receivesthe total EGR fraction setpoint 418 from the top level engine controlmodule 62 to yield an LP EGR setpoint 424.

Third, and still referring to FIG. 4, the total EGR fraction closed-loopcontrol block 406 may be any suitable closed-loop control means, such asa PID controller block or the like, for controlling the total EGRfraction. The closed-loop control block 406 includes a setpoint input406 a to receive the target total EGR fraction setpoint from the toplevel engine control module 62 and further may include a processvariable input 406 b to receive the actual total EGR fraction estimatefrom the estimator block 404. The total EGR fraction control block 406may process these inputs to generate a feedback control signal or trimcommand 406 c for summation at another arithmetic node 426 with the LPEGR setpoint 424 for input downstream at the LP EGR control block 72.Such trim adjustment may also or instead be calculated as an adjustmentto the LP EGR valve and/or exhaust throttle valve percentage openingcommand(s) and added after the LP EGR open-loop control block 72.Accordingly, the control block 406 and associate nodes would becommunicated to the open-loop control block 72 at a downstream sidethereof to adjust suitable setpoints for the valve and throttle openingpercentages.

Because the HP EGR flow may only be open-loop controlled, the LP EGRflow or fraction may be adjusted by the closed-loop control block 406 toachieve the target total EGR fraction. More specifically, becauseexhaust emissions and engine fuel economy are both highly dependent ontotal EGR fraction and to a lesser extent on the HP/LP EGR ratio, thetotal EGR fraction may be closed-loop controlled for maximum controlwhereas the HP and/or LP EGR fractions and/or the HP/LP EGR ratio may beat least partially open-loop controlled for maximum cost-effectivenessand efficiency. These open-loop control blocks 72, 74 provide goodresponse time, reduce controller interdependencies, and reduce theeffects of transients and disturbances in sensor signals. While this isone exemplary approach, other approaches are discussed below inreference to FIGS. 8-10.

Fourth, the LP and HP EGR control blocks 72, 74 may receive theirrespective LP and HP EGR setpoints in addition to the turbocharger boostpressure 409 and the engine load and speed inputs 407, 408. The LP andHP EGR control blocks 72, 74 may receive such inputs for open-loop orfeedforward control of their respective LP and HP EGR actuators. Forinstance, the LP and HP EGR control blocks 72, 74 may output LP EGRvalve and/or exhaust throttle commands 430, 432, and HP EGR valve and/orintake throttle commands 438, 440. The LP and HP EGR control blocks 72,74 may correlate HP and LP EGR flow to suitable HP and LP EGR valveand/or throttle positions using one or more models.

As shown in FIGS. 6A and 6B, the LP and HP EGR control blocks 72, 74 mayinclude various open-loop control models. For instance, the LP EGRcontrol block 72 may include any suitable model(s) 426 to correlate theLP EGR setpoint 424 to the LP EGR valve position to help achieve thetarget HP/LP EGR ratio. Also, the LP EGR control block 72 may includeany suitable model(s) 428 to correlate the LP EGR setpoint 424 to theexhaust throttle position to help achieve the target HP/LP EGR ratio.The models 426, 428 may receive any suitable inputs such as the engineload 407, the engine speed 408, and the turbocharger boost pressure 409.The models 426, 428 may be executed to generate, respectively, the LPEGR valve command 430 and/or the exhaust throttle command 432 for use byrespective actuators. Note that the actuators may operate in an openloop mode, or may be operatively coupled with any suitable sensors tomeasure actuator position and adjust the commands to achieve the targetpercentages.

Likewise, the HP EGR control block 74 may include any suitable model(s)434 to correlate the HP EGR setpoint 420 to the HP EGR valve position tohelp achieve the target HP/LP EGR ratio. Also, the HP EGR control block74 may include any suitable model(s) 436 to correlate the HP EGRsetpoint 420 to the intake throttle position to help achieve the targetHP/LP EGR ratio. Again, the models 434, 436 may receive any suitableinputs such as the engine load 407, the engine speed 408, and theturbocharger boost pressure 409. The models 434, 436 are executed togenerate, respectively, an HP EGR valve command 438 and/or an intakethrottle command 440 for use by respective actuators.

FIG. 7 illustrates a graph of exemplary LP EGR valve and exhaustthrottle opening percentages vs. target total EGR fraction. As shown,the throttle valve 42 may be substantially 100% open at about 0% EGR andstays open until about 70% EGR. The LP EGR valve 54 gradually opens fromabout 0% EGR to about 70% EGR and thereafter is substantially 100% open.A single, combined, LP EGR and exhaust throttle valve could be usedinstead of two separate valves as long as such a unitary valve devicecould substantially achieve the valve openings just described.

Referring again to FIG. 4, the turbocharger boost control block 76 maybe any suitable closed-loop control means, such as suitable PID controlblock, for adjusting turbocharger actuators to achieve a target boostpressure within safe turbo operating boundaries. The control block 76may include a setpoint input 76 a to receive boost setpoint from the toplevel engine control module 62, and an actual boost pressure input 76 bfrom the turbocharger boost sensor. The control block 76 may processthese inputs and generate any suitable turbocharger command output suchas a variable turbine geometry command 444 to adjust variable vanes ofthe turbocharger 18.

Referring now to FIG. 8, an alternative control flow 800 may be used inplace of the preferred control flow 400. This embodiment is similar inmany respects to the embodiment of FIG. 4, and like numerals between theembodiments generally designate like or corresponding elementsthroughout the several views of the drawing figures. Additionally, thedescription of the previous embodiment is incorporated by reference andthe common subject matter may generally not be repeated here.

The alternative control flow 800 may involve closed-loop adjustment ofHP EGR instead of LP EGR. In other words, an HP EGR setpoint420′—instead of an LP EGR setpoint 424′—may be adjusted to control thetotal EGR fraction. Accordingly, the closed-loop control block 406 maygenerate a control signal to adjust the HP EGR fraction—instead of theLP EGR fraction. To accommodate this change in control strategy, anoptimization block 402′ may be provided to output an LP EGR setpoint424′ instead of the HP EGR setpoint 420. Such trim adjustment may alsoor instead be calculated as an adjustment to the HP EGR valve and/orintake throttle valve percentage opening command(s) and added after theHP EGR open-loop control block 74. Accordingly, the control block 406and associate nodes would be communicated to the open-loop control block74 at a downstream side thereof to adjust suitable setpoints for thevalve and throttle opening percentages. Otherwise, the flow 800 issubstantially similar to flow 400.

Referring now to FIG. 9, a second control flow 900 may be used in placeof the preferred control flow 400. This embodiment is similar in manyrespects to the embodiment of FIG. 4, and like numerals between theembodiments generally designate like or corresponding elementsthroughout the several views of the drawing figures. Additionally, thedescription of the previous embodiment is incorporated by reference andthe common subject matter may generally not be repeated here.

In the second control flow 900, closed-loop control may be allocated toHP and LP EGR fractions in the same proportion as the HP and LP EGRsetpoints. In other words, HP and LP EGR fractions are both closed-loopadjusted in proportion to their respective HP and LP EGR setpoints.

To facilitate this change in control strategy, the closed-loop controlblock 406 may not output its trim command 406 c only to the LP EGRcontrol block 72 via the upstream arithmetic node 426 as in flow 400.Rather, the trim command may be output to both the LP and HP EGR controlblocks 72, 74. To further facilitate this change, proportionalarithmetic blocks 950, 952 may receive respective HP and LP EGRsetpoints and the total EGR setpoint 418. The proportional output fromthe arithmetic blocks 950, 952 may be received at multiplicationarithmetic blocks 954, 956 for proportional allocation of theclosed-loop trim command 406 c thereto. The multiplication outputs aresummed at downstream arithmetic nodes 426, 926 with the LP and HP EGRsetpoints for input downstream at the LP and HP EGR control blocks 72,74. Suitable checks may be implemented within the arithmetic blocks toavoid dividing by 0 when the total EGR fraction set-point is 0.Otherwise the flow 900 is substantially similar to that in flows 400and/or 800.

Referring now to FIG. 10, a third exemplary control flow 1000 may beused in place of the preferred control flow 400. This embodiment issimilar in many respects to the embodiment of FIG. 4, and like numeralsbetween the embodiments generally designate like or correspondingelements throughout the several views of the drawing figures.Additionally, the description of the previous embodiment is incorporatedby reference and the common subject matter may generally not be repeatedhere.

In the third control flow 1000, closed-loop control may be switched backand forth between the LP and HP EGR open-loop control blocks 72, 74depending on engine operating conditions at any given moment. In otherwords, either HP or LP EGR setpoints may be adjusted with closed-loopcontrol. For example, HP EGR may be closed-loop controlled to avoidturbocharger condensation when engine system temperatures are relativelyhigh, or when a rapid change in total EGR fraction is required, or whenthe turbocharger performance is less important or not required.

To accomplish the change in control strategy, a closed-loop controlblock 1006 may not provide output only to the LP EGR control block 72via the upstream arithmetic node 426 as in flow 400. Rather, the controlblock 1006 may provide output to both the LP and HP EGR control blocks72, 74. The closed-loop control block 1006 may include a setpoint input1006 a to receive the target total EGR fraction setpoint 418 from thetop level engine control module 62 and further may include a processvariable input 1006 b to receive the actual total EGR fraction estimatefrom the estimator block 404. The total EGR fraction control block 1006may process these inputs to generate alternative trim commands, an LPEGR trim command 1006 c for summation at arithmetic node 426 with the LPEGR setpoint 424 for input downstream at the LP EGR control block 72,and an HP EGR trim command 1006 d for summation at another arithmeticnode 1026 with the HP EGR setpoint 420 for input downstream at the HPEGR control block 74. The control block 1006 may be switched between thetwo outputs 1000 c, 1000 d such that the LP EGR fraction or the HP EGRfraction may be adjusted by the closed-loop control block 1006 toachieve the target total EGR fraction. Otherwise, the flow 1000 issubstantially similar to that in flows 400 and/or 800.

One or more of the various illustrative embodiments above may includeone or more of the following advantages. First, a total target EGRfraction may be allocated to HP and LP EGR paths in a manner to firstcomply with emissions regulations, and then to optimize engine fueleconomy and performance and protect and maintain an engine system.Second, use of individual total EGR, HP EGR, or LP EGR flow sensors isnot required, which sensors are costly, complicate an engine system, andintroduce failure modes. Third, one standard closed-loop control meansmay be used to control a target total EGR fraction as well as theindividual HP and LP EGR flows, thereby allowing practical andcost-effective implementation in current engine control architectures.Fourth, a combined LP EGR valve and exhaust throttle valve controlled bya single common actuator may be used and, likewise, a combined HP EGRvalve and intake throttle valve controlled by a single common actuatormay also be used.

HP/LP EGR Ratio Optimization

FIGS. 11 through 15B illustrate HP/LP EGR ratio optimization within amethod of controlling exhaust gas recirculation (EGR) in a turbochargedcompression-ignition engine system including a high pressure (HP) EGRpath and a low pressure (LP) EGR path. Referring to FIGS. 4 and 11, theoptimization block 402 may receive and process various engine systeminputs, such as engine speed, engine load, and/or total EGR fractionsetpoint, to identify and/or adjust an optimal HP/LP EGR ratio andgenerate HP and LP EGR setpoints according to that identified and/oradjusted ratio. For example, at any given moment in time duringsteady-state system operation, the sum of the LP and HP EGR setpointsmay correspond to the total EGR fraction setpoint. More specifically, ifthe total EGR fraction setpoint is 35%, then the HP EGR setpoint may be25% and the LP EGR setpoint may be 10% for a 2.5:1 ratio of HP to LPEGR. The optimization block 402 may prioritize among several differentsystem objectives or criteria, including turbocharger protection,exhaust emissions, fuel economy, and engine performance. Morespecifically, the optimization block 402 may prioritize those criteriain the order listed above according to decreasing priority.

As shown in FIG. 11, the optimization block 402 may include a ratiodetermination block 478 that receives system inputs such as engine speedand load 407, 408 and determines an HP/LP EGR ratio based on the inputs.The optimization block may also include an HP/LP EGR ratio dynamiccompensation block 480 to correct for lag time in EGR responsiveness,and arithmetic nodes 496, 497 downstream of the blocks 478, 480 togenerate LP and HP EGR setpoints. The optimization block 402 may alsoinclude any other suitable blocks, arithmetic nodes, or the like.

The ratio determination block 478 may determine the percentage of thetotal EGR fraction setpoint that will be allocated to LP EGR and to HPEGR. Because LP and HP EGR are the only two sources of EGR, theirpercentage contributions add up to 100% at least during steady-statesystem operation. For example, during cold engine operation, the ratiodetermination block 478 may allocate only about 10% of the total EGRfraction to LP EGR and about 90% of the total EGR fraction to HP EGR,which is normally warmer than LP EGR, so as to more quickly warm up theengine. During other modes of operation, the ratio determination block478 may allocate the total EGR fraction according to any other HP/LP EGRratios such as 50/50, 20/80, etc.

Referring now to FIG. 12, the ratio determination block 478 may includeseveral models, including a base model 482, which may be a model basedon steady state system operating conditions. The base model 482 mayreceive signals or values corresponding to various system operatingconditions, such as engine speed and engine load, and total EGRinformation such as total EGR setpoint. As used herein, the term“signal” includes electrical or electronic signals from sensors, valuesfrom memory, or the like. Actual parameter values may be measured from adirectly corresponding parameter sensor, or may be estimated frommeasurements from other parameter sensors or from models, and/or thelike. The base model 482 may process such input using one or moreformulas, tables, maps, or the like to generate a base EGR value, suchas a base HP/LP EGR ratio value, a base LP EGR value indicative of abase LP EGR contribution percentage, and/or a base HP EGR valueindicative of a base HP EGR contribution percentage.

The base model 482 may be developed for steady state warm engineoperating conditions and may include one or more objectives. Forexample, one objective may include maximization of fuel economy based onproviding optimal flow of air and exhaust through a turbocharger.Another objective, such as during high engine loading, may includerouting adequate amounts of exhaust gas through a turbocharger turbineso that sufficient intake manifold pressures are achieved to produceengine peak power and torque.

Although the base model 482 may receive signals indicative of any systemoperating conditions, preferably a limited set of signals is received,such as those mentioned above. This enables the base model 482 to berelatively simple and, thus, relatively reliable in comparison to modelsthat include more variables, and more complexities due to theinteractions of so many variables. For example, the model 482 may alsoinclude other variables including turbocharger speed, turbochargertemperature(s), turbocharger boost pressure, engine coolanttemperature(s), induction temperature, etc. Preferably, however, suchvariables may be used in adjustment models of the ratio determinationblock 478, such as a transient load adjustment model 484, an inductiontemperature adjustment model 486, and a turbocharger protectionadjustment model 488. The adjustment models 484, 486, 488 may outputadjustment values, which may modify or adjust the base EGR value fromthe base model 482. For example, the adjustment models may output anadjustment HP/LP EGR ratio value, an adjustment LP EGR value indicativeof an adjustment to LP EGR contribution percentage, and/or an adjustmentHP EGR value indicative of an adjustment to HP EGR contributionpercentage

Referring now to FIG. 13, the transient load adjustment model 484 may beprovided to improve a rate at which combustion gases (air and/or EGRgases) may be delivered to the engine, such as during engineacceleration. Because the amount of power and torque a diesel engine candeliver during transient operation is normally limited by the airavailable to the engine, improving the air delivery response leads tobetter engine power or acceleration responsiveness. A turbocharger isusually the primary means used to vary the delivery of air to an enginesuch as via wastegate valve adjustments, turbine or compressor vaneadjustments, or the like. But the principal factor affecting the amountof air the turbocharger compressor can pump into the engine is theamount of energy in the form of gas flow, temperature, and pressurecontained in the exhaust gas flowing through the turbocharger turbine.Because HP EGR does not power a turbocharger turbine whereas LP EGRdoes, this adjustment model 484 may produce a positive adjustment to theLP EGR value, for example, when it is determined that a target boostpressure (engine air) is higher than the actual air being delivered tothe engine.

Still referring to FIG. 13, the adjustment model 484 may include atarget load response model 484 a for determining a target turbochargerboost pressure and/or a target induction flow value, an arithmetic node484 b for comparing such target value(s) with an actual turbochargerboost pressure or induction flow value, and a turbocharger boostpressure and/or induction flow adjustment model 484 c for determining anLP EGR adjustment value based on the variance between the target andactual boost or induction value(s). The target load response model 484 amay be any formula, lookup table, map, or the like that receives systemoperating condition input such as engine load 407, or engine speed 408,or the like, and that process such input to output a correspondingtarget boost or induction flow value to the arithmetic node 484 b. Thearithmetic node 484 b may be a subtraction node that subtracts an actualboost pressure or induction flow value and passes along a differentialvalue to the adjustment model 484 c when target boost pressure orinduction flow is greater than the actual boost pressure or inductionflow. The adjustment model 484 c may receive the differential value andany other suitable system parameters and process such data with one ormore formulas, maps, tables, or the like to generate a first LP EGRadjustment value. Accordingly, the HP and LP EGR values may be modulatedtoward greater LP EGR to allow boost pressure to build more quickly.

Referring now to FIG. 14, the induction temperature adjustment model 486may be provided to modify induction temperatures by varying the HP/LPEGR ratio. This objective is usually relatively easy to obtain becausethe HP EGR temperature is usually significantly higher than the LP EGRtemperature. The adjustment model 486 may include a target inductiontemperature model 486 a for determining a target induction temperaturevalue, an arithmetic node 486 b for comparing the target value with anactual induction temperature value, and an LP EGR adjustment model 486 cfor determining an LP EGR adjustment value based on the variance betweenthe target and actual values. The target induction temperature model 486a may be any formula, lookup table, map, or the like that receivessystem operating condition input such as engine coolant temperature, orthe like, and that may be processed with such input to output acorresponding preferred induction temperature value to the arithmeticnode 486 b. The arithmetic node 486 b may be a subtraction node thatsubtracts the actual induction temperature value and passes along adifferential value to the adjustment model 486 a when target inductiontemperature is greater than the actual induction temperature. Theadjustment model 486 a receives the differential value and any othersuitable system parameters and may be processed with such data with oneor more formulas, maps, tables, or the like to generate a second LP EGRadjustment value.

The adjustment model 486 may also control induction temperature atdifferent points in an induction sub-system. For example, it may bedesirable to control induction temperature where LP EGR mixes withintake air upstream of a turbocharger compressor. This may be especiallytrue during operating conditions wherein harmful condensate can arise ifLP EGR temperature is too low. In a particular example, U.S. ProvisionalApplication No. 60/748,894, filed Dec. 9, 2005, discloses a strategy tocalculate when harmful condensate occurs and to control an LP EGR bypassvalve to avoid such conditions. The aforementioned patent application isassigned to the assignee hereof and is hereby incorporated by referenceherein in its entirety. The adjustment model 486 may use the samecalculations from the aforementioned patent application to adjust theHP/LP EGR ratio instead of or in addition to controlling the LP EGRbypass valve. Accordingly, the HP and LP EGR values may be modulated toavoid condensate in EGR coolers and/or in a turbocharger compressor, andto provide warmer induction gases to an engine.

Referring now to FIG. 15A, the turbocharger protection adjustment model488 may be provided to protect the turbocharger against overspeed andexcessive temperatures. For example, the adjustment model 488 mayinclude a turbocharger overspeed protection model 488 a that adjusts LPEGR to avoid damage to the turbocharger. The overspeed protection model488 a may receive a differential value from an arithmetic node 488 b,which may receive and compare a maximum turbocharger speed signal and anactual turbocharger speed signal. The arithmetic node 488 b may be asubtraction node, wherein the actual turbocharger speed signal may besubtracted from the maximum turbocharger speed signal to yield thedifferential value sent to the protection model 488 a. The maximumturbocharger speed signal may come from memory in a top level enginecontrol module or any other suitable location, or from some other model,or the like. The actual turbocharger speed signal may come from aturbocharger speed sensor or any other suitable speed sensor, or from atop level engine control module or elsewhere. The protection model 488 amay receive the differential value from the arithmetic node 488 b andany other suitable system parameters such as engine speed, and may beprocessed with such data to generate a third LP EGR adjustment, forexample to reduce the speed of a turbocharger.

In another example, and referring now to FIG. 15B, an alternativeadjustment model 488′ may include a turbocharger compressor tipprotection model 488 c that may adjust LP EGR to avoid damage to theturbocharger compressor. The compressor tip protection model 488 c mayreceive a differential value from another arithmetic node 488 d, whichmay receive a maximum compressor tip temperature signal and an actualcompressor tip temperature signal. The arithmetic node 488 d may be asubtraction node, wherein the actual compressor tip temperature signalmay be subtracted from the maximum compressor tip temperature signal toyield the differential value sent to the tip protection model 488 c. Themaximum compressor tip temperature signal may come from memory in a toplevel engine control module or any other suitable location, or from someother model, or the like. The actual compressor tip temperature signalmay come from a compressor temperature sensor or any other suitabletemperature sensor, or from a top level engine control module orelsewhere. The tip protection model 488 c may receive the differentialvalue from the arithmetic node 488 d and any other suitable systemparameters such as engine speed, and may be processed with such data togenerate an alternative third LP EGR adjustment value, for example toreduce the compressor tip temperature. Accordingly, the HP and LP EGRvalues can be modulated to avoid excessive system temperatures andspeeds in a turbocharger.

Any other ratio adjustment models may be provided to improve systemperformance for any other suitable parameters. For example, the LP EGRmay be further adjusted to improve aftertreament regeneration. Morespecifically, the LP EGR may be further adjusted to reduce gas flowthrough an exhaust gas aftertreatment system and/or to increase exhaustgas temperatures therethrough. Such improvements may be desirable toprovide relatively low flow and high temperatures for filterregeneration.

Referring again to FIG. 12, the LP EGR adjustment values from theadjustment models 484, 486, 488 may be communicated to arithmetic nodesdownstream of the base model 482. For example, the transient loadadjustment value may be compared to the base LP EGR value by a firstarithmetic adjustment node 490, which may be a summation node that sumsthe two signals to yield a transient load adjusted EGR value. In anotherexample, the induction temperature adjustment value may be communicatedto the transient load adjusted EGR value by a second arithmeticadjustment node 491, which may be a subtraction node that subtracts theinduction temperature value from the transient load adjusted EGR valueto yield a temperature and transient load adjusted EGR value. In afurther example, the protection adjustment value may be communicated tothe temperature and transient load adjusted EGR value by a thirdarithmetic adjustment node 492, which may also be a subtraction nodethat subtracts the protection adjustment value from the temperature andtransient load adjusted EGR value to yield a final LP EGR value.

The ratio determination block 478 may also include a limit block 494 forcomparing an LP EGR value to upper and/or lower LP EGR limits to preventinsufficient and/or excessive LP EGR levels. For example, the limitblock 494 may include an upper limit for LP EGR and/or a lower limit forLP EGR. An exemplary upper limit for LP EGR may be 90% and an exemplarylower limit for LP EGR may be 10%. Accordingly, if a final LP EGR valueincluded a 95% LP EGR, then the limit block 494 would override the valueand instead output a 90% LP EGR value. Similarly, if a final adjustedEGR value included a 5% LP EGR, then the limit block 494 would overridethat value and output a 10% LP EGR value. According to anotherembodiment, another limit block (not shown) may be provided in anysuitable location to similarly limit HP EGR.

The LP EGR value is communicated out of the ratio determination block478 via two branches; one that includes the LP EGR value, and one thatcalculates and outputs an HP EGR value. In the latter branch, the LP EGRvalue may be communicated to an arithmetic node 495, which calculates anHP EGR value from the LP EGR value. The arithmetic node 495 may be asubtraction node that subtracts the LP EGR value from a fixed value of100% to yield a corresponding HP EGR value. Accordingly, the ratiodetermination block 478 produces an LP EGR value and a corresponding HPEGR value.

Referring again to FIG. 11, the HP EGR value may be communicated fromthe ratio determination block 478 to the HP/LP dynamic compensationblock 480, which may delay the HP EGR value to account for EGR time lagin the system. During steady state system operation, a lowering of asetpoint of one of the HP and LP EGR by a given amount and a raising ofa setpoint of the other by the same amount will result in no change tothe total EGR. But there is a time lag between HP and LP EGR whereinchanges in HP EGR reach the engine before changes in LP EGR because ofthe relatively greater distance that LP exhaust gases travel compared toHP exhaust gases. In other words, because the LP EGR loop is longer thanthe HP EGR loop, changes in LP EGR take longer than changes in HP EGR.Accordingly, if the HP and LP EGR setpoints are simultaneously changedby the same amount, then the total EGR will be incorrect for a shortperiod of time. That time represents the delay between when the changein HP EGR reaches the engine and when the change in LP EGR reaches theengine.

In a specific example, if a total EGR of 20% is split 50/50 between HPand LP EGR, then both HP and LP EGR would be 10%. If the HP/LP EGR ratiowas changed to 40/60, then HP EGR would decrease to 8% and LP EGR wouldeventually increase to 12% to yield the 20% total EGR fraction over thelong term. But over a shorter term, while the HP EGR would decrease to8% relatively quickly, the LP EGR would increase relatively slowly andthe engine may see less than the 12% LP EGR for some time. Hence, theengine would temporarily experience less than the 20% total EGR,somewhere between 18%-20% total EGR. In other words, the engine wouldexperience a drop in total EGR for a short period of time with aconcomitant effect on emissions performance.

Therefore, the dynamic compensation block 480 may correct for suchtransients in total EGR fraction upon changes in HP/LP EGR ratio,because the total EGR fraction tends to be of higher priority than theHP/LP EGR ratio to maintain exhaust emissions at as low a level aspossible at all times. Accordingly, the dynamic compensation block 480may ensure that changes to the HP/LP EGR ratio are carried out asquickly as possible while substantially maintaining the total EGRfraction. More specifically, the compensation block 480 may delaydownstream communication of the HP EGR value so that changes in HP andLP EGR arrive at the engine at substantially the same time.

For example, the compensation block 480 may carry out a fixed delay thatis equivalent to a typical delay between changes in HP and LP EGRarriving at an engine. But because the delay may vary depending onvarying system operating conditions, the delay may be variable accordingto a model. In any case, although the delay may temporarily yield aninaccurate HP/LP EGR ratio, it would allow the LP EGR to arrive at theengine at substantially the same time as the HP EGR arrives so as toensure a substantially constant total EGR fraction. The compensationblock 480 could be included elsewhere, such as further downstream in thecontrol flow, but response time may not be as fast.

Referring still to FIG. 11, the LP EGR and HP EGR values may then becommunicated to respective arithmetic blocks 496, 497, which may bemultiplication blocks that may multiply the LP EGR value and the HP EGRvalue by the total EGR fraction setpoint 418. The LP EGR setpoint may bedetermined by multiplying the LP EGR value by the total EGR fraction. Ina specific example, a 10% LP EGR can be applied to a total EGR fractionof 20% to yield a 2% LP EGR setpoint (and, conversely, an 18% HP EGRsetpoint). If the 20% total EGR increased to 30%, then the same HP/LPEGR ratio would yield a 3% LP EGR and a 27% HP EGR. But, as set forthabove, application of the target total EGR fraction to the HP EGR valuemay be delayed relative to application of the target total EGR fractionto the LP EGR value to account for the lag time between LP and HP EGR.

Although the ratio determination block 478 has been described withrespect to a base LP EGR value and LP EGR adjustment values, otherembodiments may be equivalent. For example, the ratio determinationblock 478 may also or instead include a base model to generate an HP EGRbase signal, and adjustment models to generate HP EGR adjustment values.In another example, the ratio determination block 478 may also orinstead include a base model to generate an HP/LP EGR ratio base signal,and adjustment models to generate HP/LP EGR ratio adjustment values.

According to another embodiment, one or more of the adjustment models484, 486, 488 may be disabled at any given time and/or under any givensystem operating conditions. For example, once the engine is runningunder normal operating temperatures, the induction temperatureadjustment model 486 could be disabled. Exemplary normal operatingtemperature ranges may include 75-85° C. for engine coolant, 70-110° C.for engine oil, and 10-50° C. for engine intake gas.

According to a further embodiment, one or more of the adjustment models484, 486, 488 may override one or more of the other adjustment models.For example, if a protection adjustment value exceeds a predeterminedmagnitude, then the transient load adjustment model 484 may be disabled.

Although the exemplary systems and methods have been described inconjunction with a typical HP/LP EGR architecture, any suitable two ormore EGR path architecture may be used. For example, the EGRarchitecture may include an engine internal HP EGR flow path, a dualstage turbo EGR flow path, EGR flow paths without coolers, and/or thelike.

The method(s) or any portion thereof may be performed as part of aproduct such as the system 10 of FIG. 1, and/or as part of a computerprogram. The computer program may exist in a variety of forms bothactive and inactive. For example, the computer program can exist assoftware program(s) comprised of program instructions in source code,object code, executable code or other formats; firmware program(s); orhardware description language (HDL) files. Any of the above may beembodied on a computer usable medium, which include storage devices andsignals, in compressed or uncompressed form. Exemplary computer usablestorage devices include conventional computer system RAM (random accessmemory), ROM (read only memory), EPROM (erasable, programmable ROM),EEPROM (electrically erasable, programmable ROM), and magnetic oroptical disks or tapes.

The above description of embodiments of the invention is merelyexemplary in nature and, thus, variations thereof are not to be regardedas a departure from the spirit and scope of the invention.

1. A method of controlling exhaust gas recirculation (EGR) in aturbocharged compression-ignition engine system including a highpressure (HP) EGR path and a low pressure (LP) EGR path, the methodcomprising: determining a target total EGR fraction for compliance withexhaust emissions criteria; and determining a target HP/LP EGR ratio tooptimize other engine system criteria within the constraints of thedetermined target total EGR fraction, and including: using at leastengine speed and load as input to a base model to output a base EGRvalue; using at least one other engine system parameter as input to atleast one adjustment model to output at least one EGR adjustment value;and adjusting the base EGR value with the at least one EGR adjustmentvalue to generate an adjusted EGR value.
 2. The method of claim 1,wherein the at least one adjustment model includes a transient loadadjustment model for adjusting the base EGR value to improve a rate atwhich combustion gases can be delivered to an engine.
 3. The method ofclaim 2, wherein the transient load adjustment model includes:determining a target turbocharger boost pressure value; comparing thetarget turbocharger boost pressure with an actual turbocharger boostpressure value; and determining the at least one EGR adjustment valuebased on the variance between the target and actual boost pressurevalues.
 4. The method of claim 2, wherein the transient load adjustmentmodel includes: determining a target induction flow value; comparing thetarget induction flow value with an actual induction flow value; anddetermining the at least one EGR adjustment value based on the variancebetween the target and actual induction flow values.
 5. The method ofclaim 1, wherein the at least one adjustment model includes an inductiontemperature adjustment model for modifying induction temperatures. 6.The method of claim 5, wherein the induction temperature adjustmentmodel includes: determining a target induction temperature value;comparing the target induction temperature value with an actualinduction temperature value; and determining the at least one EGRadjustment value based on the variance between the target and actualinduction temperature values.
 7. The method of claim 1, wherein the atleast one adjustment model includes a turbocharger protection adjustmentmodel for protecting a turbocharger against at least one of overspeed orexcessive temperatures.
 8. The method of claim 7, wherein theturbocharger protection adjustment model includes: comparing a maximumturbocharger speed signal and an actual turbocharger speed signal toyield a differential speed signal; and processing the differential speedsignal with at least one other engine system parameter besidesturbocharger speed to generate the at least one EGR adjustment value. 9.The method of claim 7, wherein the turbocharger protection adjustmentmodel includes: comparing a maximum turbocharger compressor tiptemperature signal and an actual turbocharger compressor tip temperaturesignal to yield a differential temperature signal; and processing thedifferential temperature signal with at least one other engine systemparameter besides turbocharger compressor tip temperature to generatethe at least one EGR adjustment value.
 10. The method of claim 1,wherein the other engine system criteria include turbochargerprotection, fuel economy, and engine performance, and wherein EGR isadjusted to prioritize among the criteria in the following order ofdecreasing priority: turbocharger protection; exhaust emissions; fueleconomy; and engine performance.
 11. The method of claim 1, wherein oneor more of the adjustment models is disabled under predetermined systemoperating conditions.
 12. The method of claim 11, wherein an inductiontemperature adjustment model is disabled when an engine is running undernormal operating temperatures.
 13. The method of claim 1, wherein one ormore of the adjustment models overrides one or more of the otheradjustment models.
 14. The method of claim 13, wherein a transient loadadjustment model is disabled if a turbocharger protection adjustmentvalue exceeds a predetermined magnitude.
 15. The method of claim 1,further comprising: comparing the adjusted EGR value to at least one ofan upper or lower EGR limit to prevent at least one of excessive orinsufficient EGR.
 16. The method of claim 1, wherein the adjusted EGRvalue is an adjusted LP EGR value.
 17. The method of claim 16, furthercomprising: calculating an HP EGR value using the adjusted LP EGR value;and delaying downstream communication of the HP EGR value to account forEGR time lag between HP and LP EGR.
 18. The method of claim 17, furthercomprising: applying the target total EGR fraction to the HP EGR valueand the adjusted LP EGR value to generate HP and LP EGR setpoints.
 19. Amethod of controlling exhaust gas recirculation (EGR) in a turbochargedcompression-ignition engine system including a high pressure (HP) EGRpath and a low pressure (LP) EGR path, the method comprising:determining a target total EGR fraction for compliance with exhaustemissions criteria; determining a target HP/LP EGR ratio to optimizeother engine system criteria within the constraints of the determinedtarget total EGR fraction by using at least engine speed and load in atleast one model to output an LP EGR value and an HP EGR value; applyingthe target total EGR fraction to the LP and HP EGR values to establishLP and HP EGR setpoints; and delaying downstream communication of the HPEGR value to account for lag time between LP and HP EGR.
 20. The methodof claim 19, wherein the delaying step includes delaying application ofthe target total EGR fraction to the HP EGR value.
 21. A computer usablemedium embodying instructions executable by a processor to enable themethod of claim
 1. 22. A computer usable medium embodying instructionsexecutable by a processor to enable the method of claim
 19. 23. Aproduct including apparatus for implementing the method of claim
 1. 24.A product including apparatus for implementing the method of claim 19.